Crate BV120H (older blue tolex) noise problem

[Danglin' Fury]

Greetings fellow worker bees and electronics wizards! I just had ANOTHER one of these come to my bench (yaaaaaay) So everything except V1 is lighting up. No sound when plugged into the input. So I turned the effects loop on and plugged into the effects return. Bam! I got sound! And pretty decently clean luscious tube sound for a BV. Anyway. So I take the preamp tubes out and I noticed some cracked solder at pins 4 and 5 so I reflowed all the tube socket pins. Aaaaaand we have a working preamp now! BUT!!! There is this.... unGodly loud white noise/ hum that only gets louder as one increases the volume. (As is expected) When I plug into the effects return it isnt there. And, plugged into the input, the clean channel in the preamp sounds....anemic compared to direct into the effects return. Any ideas? Has anyone seen this before? The dirty channel sounds good but that white noise/hum is just way too overbearing. Dude that owns it says he can just use a noise gate but I REALLY dont want to give him back an amp that is obviously not fully repaired. Thanks for looking guys! And gals!

[Jazz P Bass]

If the Volume control affects the noise then the issue is before it.
V1 & it's associated parts is all there is.
Which includes the input jack.

[Helmholtz]

So you have white noise plus hum?

Does the noise stop when you pull V1?

Schematic please.

[TomCarlos]

I am attaching a schematic for the BV120Hb.

BV120Hb.pdf

[Danglin' Fury]

I agree. If the volume affects it, then it is pre volume. Also, I came across a mod page for this thing, and it recommends completely removing the Sidac that is in the schematic right in the heater circuit. Dude says it can and does cause hum and white noise. Recommend, or no?

[Helmholtz]

A good SIDAC should have no effect at all below its breakover voltage (120V). That high voltage across the heater winding would require a lightning stroke.

If you're curious, lift one end and see if it makes a difference.

What resistance do you measure between V1 pin2 and ground with nothing plugged into input?

[Danglin' Fury]

I might try lifting it. Would unplugging and re-plugging the speaker while shifting the impedance do that? Apparently dude did that. I just talked to him. I am FULLY aware that is bad. Very bad. But would that have killed the sidac? It didn't affect the OT apparently. The power tubes sound quite delicious.

[Helmholtz]

I might try lifting it. Would unplugging and re-plugging the speaker while shifting the impedance do that? Apparently dude did that. I just talked to him. I am FULLY aware that is bad. Very bad. But would that have killed the sidac? .

I can't see how that could increase heater to ground voltage to 120V. OTOH, lightning induced mains spikes do happen, though very, very rarely.
Also a fault in the heater supply might cause buzz but not white noise.
Check if the R106/R107 are good and measure DCFIL1/2 voltages.

But I actually suspect your input jack switching contacts.

[g1]

Sorry if I missed it but I didn't see if you tried a different tube in V1 position?
I had one of these with a SIDAC problem, I think it was hum, but some kind of noise. Like Helmholtz said, just disconnect one of the legs to see if it is the cause of the noise.

[Danglin' Fury]

I tried a different tube in V1... no joy. I replaced the input jack... no joy. It sounds like... white noise bit it ebbs and flows, if that makes any sense? I can hear it with all the volume controls off and even when I pull the cable out of the input. What about those opto isolators? Why do they even have those in there? Could the one around V1 be causing my issue?

[Helmholtz]

No answers - no fun.

I prefer a methodical approach by eliminating possible causes starting from the input.

[Danglin' Fury]

No answers - no fun.

I prefer a methodical approach by eliminating possible causes starting from the input.

This is the way. I was just hoping someone had a similar issue and could steer me in a good direction. Thank you for your poignant responses. You are a great help. As is everyone on this forum. Plus I loaned my o-scope to a "friend" and he has not yet returned it. That would help a ton. Lol

[Helmholtz]

I will continue when I have answers to my pending questions.

Also it's not clear how control settings influence the noise. Is it silent with volumes a zero?

I could imagine that a leaky coupling cap causing DCV across an Opto LDR might cause strange noises.

[Danglin' Fury]

Sorry man. I have been running ragged between my day job and my Mom breaking her hip, I'm just now seeing all the questions you asked. I will most certainly have the answers to said questions after I leave work today. I will have a meter in that amp around 6:30. P.M. Central time (U.S.) and thank you again for all your help.

[Danglin' Fury]

I will continue when I have answers to my pending questions.

Also it's not clear how control settings influence the noise. Is it silent with volumes a zero?

I could imagine that a leaky coupling cap causing DCV across an Opto LDR might cause strange noises.

I was thinking the same thing about the leaky cap. But the noise is NOT silent with the volumes at zero. It is still present. It seems to also be present when I remove the guitar cable from the input. It is more quiet when in this state, but still audible.